What Each Complication Actually DoesThe annual calendar was Patek Philippe's invention in 1996. It uses a mechanism that distinguishes between 30- and 31-day months but treats February like any other short month. With the annual calendar, you have one correction per year, at the end of February. For most owners, this is a small concession for a great value.The perpetual calendar goes further. It carries a mechanical model of the Gregorian calendar, including the four-year leap year cycle, and adjusts itself without input. Under normal wear, it never needs a date correction. The catch is what happens when it stops. A perpetual that runs down or sits in a drawer requires a careful reset with the crown and correctors in the right sequence. Force the hands at the wrong calendar position, particularly between 8 PM and 3 AM, when the mechanism is mid-change, and you risk damaging the gears and levers responsible for the calendar advance.